Dow Company presented [Me2Si(Me4C5)NtBu]TiCl2 (Constrained-Geometry Catalyst, hereinafter abbreviated as CGC) in the early 1990's (U.S. Pat. No. 5,064,802). Compared to the previously known metallocene catalysts, the advantages of CGC in the copolymerization reaction of ethylene and alpha-olefin can be summarized as follows: (1) it exhibits high activity even at high polymerization temperature, and simultaneously, produces high molecular weight polymer, and (2) it also has very excellent copolymerizability with alpha-olefins having large steric hindrance such as 1-hexene and 1-octene. In addition, as various properties of CGC in polymerization reactions have been gradually known, there have been vigorous attempts in academic and industrial fields to synthesize derivatives thereof and use them as polymerization catalysts.
A Group 4 metallocene compound having one or two cyclopentadienyl groups as ligand may be activated with methylaluminoxane or a boron compound to be used as a catalyst of olefin polymerization. Such catalyst exhibits unique properties which may not be realized by Ziegler-Natta catalysts of the prior art.
Specifically, a polymer obtained using the catalyst has narrow molecular weight distribution and better reactivity to second monomers such as alpha-olefin or cyclic olefin, and the distribution of the second monomers of the polymer is uniform. And, when alpha olefin is polymerized, stereoselectivity of the polymer may be controlled by changing substituents of the cyclopentadienyl ligand in the metallocene catalyst. In addition, when ethylene is copolymerized with other olefins, the degree of copolymerization, molecular weight, and the distribution of second monomers and the like may be easily controlled by changing substituents of the cyclpentadienyl ligand in the metallocene catalyst.
Meanwhile, since metallocene catalysts are expensive compared to Ziegler-Natta catalysts of the prior art, they may have economical value when they have good activity. If reactivity to second monomers is good, polymer including many second monomers may be obtained even with a small amount of second monomers introduced.
According to the results of studies on various catalysts by many researchers, it was proved that bridged catalysts generally have good reactivity to second monomers. Bridged catalysts which have been studied may be classified into three kinds according to the shape of the bridge. The first one is a catalyst wherein two cyclopentadienyl ligands are connected by an alkylene dibridge by the reaction of electrophile such as alkyl halide with indene or fluorene and the like, the second one is a silicon-bridged catalyst connected by —SiR2—, and the third one is a methylene-bridged catalyst obtained from the reaction of fulvene with indene or fluorene and the like.
However, among these attempts, a few catalysts are practically applied in commercial process, and there has been continuous demand for the preparation of catalysts exhibiting more improved polymerization performance.